You can hear the drone before it’s visible, whining like a mosquito above the hillside grounds of Rwanda’s Kabgayi District Hospital. Emerging through a patch of fog, roughly 100 feet in the air, the small plane quickly disappears again, circling in an oblong pattern as it descends toward an altitude low enough to make its drop. After a period of silence, it’s suddenly back, swooping over the roof of Kabgayi’s accident ward to drop its payload on the driveway with a thud. On the ground lies a red cardboard container, roughly the size of a shoebox, attached to a parachute made of wax paper and biodegradable tape.

The contraption may resemble a children’s art project, but its contents are lifesaving. Packed tightly inside are two units of human blood, which will probably soon be used for transfusions during surgeries or complicated childbirths, or to treat young victims of malaria...